The present invention concerns functional overlays which are not fastened over their whole surface and are attached to a flexible object in such a manner that they do not form a fold when the object is bent towards the side on which the overlay lies, and it concerns functional overlays to cover analyte-sensitive fields of test strips which ensure a uniform distribution of applied analyte-containing samples even when the test strips have more than a single detection field. The invention additionally concerns test strips which are furnished with such overlays and their use to determine diagnostically relevant analytes.
Functional overlays in the sense of this description of the invention are those which have a function without which the intended use of the object to which they belong would be impossible.
So-called carrier-bound tests are often used for the qualitative or quantitative analytical determination of components of body fluids in particular of blood. In these the reagents are present on or in corresponding layers of a solid test carrier which is contacted with the sample. The reaction of liquid sample and reagents leads to a detectable signal especially to a change in colour which can be evaluated visually or with the aid of an instrument, usually by reflection photometry.
Test carriers are frequently in the form of test strips which are essentially composed of an oblong support layer made of plastic material and detection layers mounted thereon as test fields. However, test carriers are also known which are designed as small quadratic or rectangular plates. In the following description the term xe2x80x9ctest stripsxe2x80x9d is also intended to encompass test carriers which do not have a strip shape.
Test carriers of the above-mentioned type are for example known from the German Patent document 21 18 455. This describes diagnostic test carriers for the detection of analytes in liquids which are composed of a support layer and at least one detection layer containing the detection reagents which is provided with a cover layer on its surface that does not adjoin the support layer. The cover layer can be composed of a fine-meshed network in the form of a fabric, knitted fabric or fleece. Plastic fabrics are stated as preferred networks in order to achieve a rapid wetting of the detection layer with sample liquid and to avoid interfering chromatographic effects. In order to detect an analyte in a liquid, such a diagnostic test carrier is dipped into a corresponding liquid preferably urine. In this manner the detection layer comes into contact with a very large excess of liquid which cannot be taken up by the test carrier. However, depending on the duration of contact of the detection layer with the liquid to be examined different colour intensities can be observed.
As a rule the results obtained are more positive the longer the contact time is. Therefore a correct quantitative analyte determination is not possible in this manner when there is a large excess of sample.
On the other hand a sample volume that is too small for a test carrier construction is a frequent cause for false measured values in diabetes monitoring i.e. the regular control of the blood of diabetics for the content of glucose.
Test carriers with the smallest possible volume requirement are therefore the goal of diverse current developments.
Diagnostic test carriers in the form of test strips which offer a considerable advance with regard to reproducibility of the test results even when different sample volumes are applied and with regard to hygienic handling are known from EP-A-0 821 233.
They contain a support layer with a detection layer arranged thereon containing reagents required to determine the analyte in a liquid sample and an overlay made of a network covering the detection layer which is larger than the detection layer and is attached to the support layer on both sides of the detection layer preferably via a spacer and in contrast rests directly on the detection layer without attachment i.e. essentially is in contact with the whole surface of this. The network used as an overlay should be hydrophilic but not alone be capillary active. An inert cover made of sample-impermeable material is arranged over the areas of the overlay which extend beyond the detection layer in such a manner that a sample application site remains free on the area of the network covering the detection layer.
In this construction the network covering the detection layer is preferably composed of high denier, relatively coarse-meshed monofilament fabric with an adequately large mesh size so that liquid can pass through the net onto the detection layer (page 3, line 12). In the example of this application a monofilament fabric with a mesh size of 280 xcexcm is used. It has an important function: it rapidly passes sample liquid applied to its surface onto the underlying detection layer. When the detection layer is saturated a sample excess which may be present is led away into the boundary regions of the network which extend beyond the detection layer. In this manner small amounts of sample are made completely available to the detection layer but longer exposure to a sample excess which can lead to false-positive results is avoided.
An embodiment is also described in this document which has two or several detection layers arranged next to one another which are intended for the measurement of the same or different analytes.
The use of test carriers of this design of course at first sight offers tempting advantages with regard to work and cost savings since it should enable two or even several measurements to be carried out with a single application of sample. However, in practice various difficulties arise when it is attempted to use this design. An essential requirement that the required minimum sample volume of 6 xcexcl for the measurement should not be exceeded cannot be fulfilled with such a test carrier construction in which the detection fields each have an area of about 20 to 40 mm2 and the said network extends over both detection fields. It turns out namely that there is only a wetting of the test field above which the sample is applied. The second test field is not wetted even when sample volumes of up to 30 xcexcl are applied. Even when the attempt to wet both test fields by placing the fields directly next to one another on the support without a gap does not lead to success. It turns out that even the tiny distance between the test fields of 5 to 10 xcexcm which is still present in this case prevents blood from xe2x80x9cjumpingxe2x80x9d from one to the other field.
Both test fields are only wetted when the sample is applied exactly over the gap between the two test fields. However, such an exact sample application is difficult to accomplish in practice. Although both test fields can be wetted when an application slit of only ca. 2 to 3 mm in width is left free in the inert cover above the network over the gap between the measurement fields a minimum sample volume of over 15 xcexcl is required in this construction to completely wet the test fields.
It was found that the sample volume required for measurement can be reduced to ca. 4 xcexcl when a functional overlay with a matched liquid distribution capacity (spreading capacity) preferably made of a thin textile material composed of low denier monofilaments or corresponding multifilament yarns is incorporated into the test construction instead of the high denier, coarse-meshed network.
Thin textile spreading fabrics, knitted spreading fabrics or spreading fleeces which have permeability, conductivity and capillary activity matched to the requirements of the individual case and usually have a low storage capacity are thus considerably more suitable than the high denier networks recommended in EP-A-0 821 233 especially for spreading low sample volumes over two or several different detection fields. It was also found that for perfect spreading of the analyte it is necessary that the spreading overlay should be in good contact with the base over which the spreading is to occur.
This requirement now results in a problem when using thin functional overlays which require a good contact with the underlying layer for perfect function if the test strips are inserted into a measuring instrument such as the well-known GLUCOTREND(copyright) instrument for evaluation in which it is placed under mechanical tension by bending to position it exactly. If the bending occurs towards the side on which the functional overlay according to the invention is located, then this has a tendency to form a fold in the area of the detection layer where the overlay rests free and unattached i.e. to lift up from the detection layer which considerably impairs or prevents the uniform filling of the detection layer. Especially in the case of test strips with two adjacent test fields a uniform supply to both test fields is prevented when the sample is applied.